Description
在我们请求某个 URL 时，应用程序是如何对不同的 URL 作出不同的响应的？在 Flask 里面又是怎么实现的？在前面看见 Flask 类里面初始化了一个实例变量 url_map，这是个 werkzeug.routing.Map 对象，实际上可以把它看作是个 路由映射规则 ，类似于字典这样的数据结构，里面存的都是 Web 应用的各种路由规则，供我们使用。
在 Flask 里面route装饰器估计是最常用最熟悉的了，下面就来看看这个神奇的东西背后都有哪些幺娥子，先找到了Flask.route方法实现，如下
class Flask(object)
# ...
def route(self, rule, **options):
"""
By default a variable part
in the URL accepts any string without a slash however a different
converter can be specified as well by using ``<converter:name>``.
=========== ===========================================
`int` accepts integers
`float` like `int` but for floating point values
`path` like the default but also accepts slashes
=========== ===========================================
An important detail to keep in mind is how Flask deals with trailing
slashes. The idea is to keep each URL unique so the following rules
apply:
1. If a rule ends with a slash and is requested without a slash
by the user, the user is automatically redirected to the same
page with a trailing slash attached.
2. If a rule does not end with a trailing slash and the user request
the page with a trailing slash, a 404 not found is raised.
The :meth:`route` decorator accepts a couple of other arguments
as well:
:param rule: the URL rule as string
:param methods: a list of methods this rule should be limited
to (``GET``, ``POST`` etc.). By default a rule
just listens for ``GET`` (and implicitly ``HEAD``).
:param subdomain: specifies the rule for the subdoain in case
subdomain matching is in use.
:param strict_slashes: can be used to disable the strict slashes
setting for this rule. See above.
:param options: other options to be forwarded to the underlying
:class:`~werkzeug.routing.Rule` object.
"""
def decorator(f):
self.add_url_rule(rule, f.__name__, **options)
self.view_functions[f.__name__] = f
return f
return decorator这里还能观察到一点小技巧：如果添加的路由规则由/结尾，当用户请求该URL时会自动重定向至带/的URL，相反如果添加的没有/结尾，当请求/结尾的URL时直接返回404，还说明了这个版本的路由匹配规则里converter一共有三种类系，int，float，path。实际上发现，在route内部也是通过Flask.add_url_rule 方法来实现路由规则添加，（所以我们大可以使用 add_url_rule 来实现与route装饰器一样的功能，但是，为了使代码更 Pythonic 更简洁，兼顾可读性，在两种方法都可行的情况下更建议使用route装饰器来添加路由规则）。跟踪到 Flask.add_url_rule：
class Flask(object):
# ...
def add_url_rule(self, rule, endpoint, **options):
"""Connects a URL rule. Works exactly like the :meth:`route`
decorator but does not register the view function for the endpoint.
Basically this example::
@app.route('/')
def index():
pass
Is equivalent to the following::
def index():
pass
app.add_url_rule('index', '/')
app.view_functions['index'] = index
:param rule: the URL rule as string
:param endpoint: the endpoint for the registered URL rule. Flask
itself assumes the name of the view function as
endpoint
:param options: the options to be forwarded to the underlying
:class:`~werkzeug.routing.Rule` object
"""
options['endpoint'] = endpoint
options.setdefault('methods', ('GET',))
self.url_map.add(Rule(rule, **options))除了给了一些常规用法外，最关键的是方法的最后一句 self.url_map.add(Rule(rule, **options))，所以最后都是添加到 url_map 里面，那么这个 url_map 和 Rule 又是什么东西？跟踪到 werkzeug.routing，他们的代码都相对比较长，可以看看这里 Rule，Map，先直接看看他们的 demo 吧。
>>> m = Map([
... Rule('/', endpoint="index"),
... Rule('/about/', endpoint="/about"),
... Rule('/help', endpoint="help")
... ])
>>> c = m.bind("example.com", '/')
>>> c.build('index')
'/'
>>> c.match("/")
('index', {})
>>> c.match('/about')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/local/lib/python2.7/dist-packages/werkzeug/routing.py", line 1524, in match
safe='/:|+') + '/', query_args))
werkzeug.routing.RequestRedirect: 301: Moved Permanently
>>> c.match('/help')
('help', {})
>>> m.add(Rule('/view/<int:post>/<int:page>', endpoint="view"))
>>> c.match('/view/22/33')
('view', {'post': 22, 'page': 33})
>>> 从这几个例子里面便可以看见 werkzeug 的 Map，Rule 主要功能了，先添加路由规则，再绑定，之后便可以进行匹配了。重现了前面提到的 URL 带不带反斜杠问题。 werkzeug 的路由过程最后都是转换到 endpoint，再返回给用户。关于路由暂时先说这些，后面还有一些更重要的细节。
欲知后事如何，请听下回分解。
Activity